Field of the Invention
This invention relates to frequency scanning laser systems. More particularly, it relates to tunable broadband laser systems and apparatus for continuously varying the output radiation frequency of such a laser system with a calibrated scan width.
In recent years tunable broadband lasers such as dye lasers have attracted much attention as a research tools because of their property of being able to provide output wavelengths over a comparatively large segment of the visible spectrum. The basic structure and manner of operation of dye lasers has now become well known and is disclosed in published literature, including U.S. Pat. No. 3,873,941 to Yarborough et al.
With the advent of the ability to scan the output frequency of the laser a new dimension in precision spectroscopy was provided by illuminating objects of interest with the output radiation from such a laser as it was scanned over a range of output frequencies. However, for maximum scientific usefulness, it became necessary not merely to scan the laser output frequency but to do so in a repeatable manner and over a known range of output frequencies. Such capability is particularly necessary where a broad scan width of, for example, 30 GHz is initially scanned to determine the presence or absence of spectroscopic events within the scan range, and then much narrower identified portions of that scan range are investigated in detail with a narrow scan width. To obtain the desired scan it thus became necessary to develop apparatus and techniques for selecting carefully controlled scan widths and maintaining those scan widths accurately calibrated over the entire operating spectral range of the dye laser from blue (about 400 nanometers wavelength) to red (about 800 nanometers wavelength). The gross variations in output radiation frequency for defining the general spectral region to be scanned may be effected in a conventional manner using conventional apparatus such as the birefringent filter disclosed in U.S. Pat. No. 3,868,592 to Yarborough et al. However, once the general spectral region to be scanned has been selected by such means, it is necessary that the scanning of the laser system be carried out in a more controlled, calibrated manner to provide a high resolution, narrow spectral width scan. One means for achieving controlled scanning is by slaving tuning means within the laser cavity, such as a tipping Brewster plate and one or more etalons, to a tunable optical frequency reference cavity such as a Fabry-Perot interferometer containing a path length varying means, such as a tipping Brewster plate, which serves to vary the optical path length of this reference cavity, as is disclosed in the paper entitled "Frequency Stabilization of a CW Dye Laser" by Grove, Wu and Ezekial, Proceedings of the SPIE, Vol. 49, pp. 75-79 (1975) and in the article "Direct Optical Measurement of Sodium Hyperfine Structure Using a CW Dye Laser and an Atomic Beam" by Schuda, Hercher and Stroud, 22 Applied Physics Letters No. 8, pp. 360-62 (1973). However, while this structure provides for some repeatability in frequency scanning at a specific laser nominal output radiation frequency, it has a serious deficiency for scientific purposes in its lack of any provision for calibration and adjustment of the scan width as is required when operating the laser in substantially different spectral regions.
The necessity for such calibration and adjustment stems from the manner of scanning the reference cavity and laser cavity, whereby the optical path lengths of those cavities are varied slightly, thus varying the resonant frequency of the cavity and causing the output radiation from the cavity to scan in synchronism with the cavity length changes. Inherently, when the cavity length is changed by one-half wavelength of the radiation into the cavity, the change in the resonant frequency of the cavity is equal to c/21, where c equals the speed of light and 1 is the length of the cavity. By the very nature of the interferometer reference cavity only light having a frequency equal to the sum multiple of the cavity resonant frequency is transmitted out of the cavity. Thus, any change in optical length of the reference cavity will cause transmission through the cavity of a different wavelength, or frequency, of light, and a continuous variation of the cavity optical length will affect a continuous or scanning variation in the wavelength transmitted. Since the actual wavelength of the radiation is inversely proportional to its frequency and varies from one spectral region to another (and by a 2:1 ratio when going from blue at 400 nm to red at 800 nm), the amount of path length change must be recalibrated for each spectral region to obtain the proper incremental path length change to maintain the proper relationships in the relative movements of the various laser cavity scan controlling elements and the reference cavity scanning member and to obtain the desired scan width in that spectral region.